Intermittently operating automatic frequency control



March 9, 1948. s. E; MILLER INTERMITTENTLY OPERATING AUTOMATIC FREQUECY CONTROL Filed oct. 11, 1944 )NVE/v70@ Y 5.5. MILLER' Y A 7' TOR/VE Y Patented Mar. 9, 1w948 ff I 2,437,268

INTERMITTENTLULr OPERATING AUTOMATIC FREQUENCY CONTROL Stewart E. Miller, Jackson Heights, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application october 11', 1944, serial No. 558,249

(c1. 25o-1s) 9 Claims.

This invention relates to automatic frequency control for radio receivers of the radar or other pulse reflection type systems.

In one type of radio object location or radar system pulses of high frequency radiations are transmitted and the resultant reflections from the objects under scrutiny received and observed. By the use of directional transmitting andreceiving systems the angular location of the target may be determined and by means of systems for timing the transmitted and reected pulses the distance to the target can be ascertained. It is often found advantageous in such equipment to employ a single antenna system for both transmitting and, receiving. When this is done some stem for prctecting the receiver from the effects of the highV energy transmitted pulses is desired. One method of accomplishing this resultwhich has been found highly satisfactory is by the use in the path to the receiver of a device resonant at the transmitted radio frequency and shunted by a gas discharge gap. Thel high energy of the transmitted pulse will break down this gap and effectively short-circuit the path to the receiver for such oscillations While the low energy reflected pulses will not affect the gap and will pass unattenuated to the receiver.

It has also been found desirable to use automatic frequency control of the receiver beating oscillation in such systems. This is' particularly true when they are operated at very high frequencies Where the frequency stability of both the transmitter and receiver oscillators may not be of a very high order. For various reasons including that of avoiding interference from both friendly systems of the same type and from deliberate enemy attempts at jamming it has been founddesirable to operate this automatic frequency control from the transmitted Apulses rather than from received signals. In other words the receiver is automatically tuned to theA frequency of the transmitted pulses and so has a maximum response to reflections thereof. TWO preferredl systems havelbeen used for this purpose. In the first, part of the high frequency energy of the pulse is taken directly from the transmitter independent'of the normal signal path to the receiver. In the second, the energy leaking through the gas tube duplexing unit described above is used for this purpose. With this second type of system the automatic frequency control system is rendered inoperative except during the periods of the transmitted pulse so that it is not operated by received signals but only by the oscillations from its own transmitter.

The first system has quires radio frequency circuits including a first detector which substantially duplicates the radio frequency circuits of the normal receiver path but are additionalfthereto. The second system does not have such a limitation as the automaticl frequency control circuit is branched off the normal receiver circuit at the intermediate frequency level. However, with this second system difficulties arel sometimes encountered as a result of the fact that the gas tube duplexing unit requires aV finite time to come into operation.. Accordingly, the receiver circuit may besubjected to a spurt of high energy oscillations during the beginning of a transmitted pulse. The effect of this in both the duplexing unit and the receiver circuit may be to produce spurious frequencies and consequently instability or false operation of the automatic frequency control in so far as the normal transmitter frequency is concerned.

An object of this invention is to improve the operation of automatic frequency control for radar systems.

A further object of the invention is tovelimlnate the effects of initial high 'energy leakage through the duplexing unit of radio pulsing'systems. Y

In accordance with a feature of this invention a control voltage is supplied to the radio receiver circuit to block it during the initial portion o the transmitted pulse. 'Ihis renders the receiver inoperative until the discharge gap of the duplexing unit has had time to break down, after which the attenuation produced by the discharge isV the receiver is included in the receiver circuit but is made inoperative except during the time that pulses are transmitted so that the receiver will be tuned for maximum reception at the frequency of the transmitted pulses. A control pulse is produced by differentiating the modulating pulse applied to the transmitter and this is used to block the'radio receiver circuit during the initial portion of the transmitted pulse.

the limitation that it re-y tween transmitted pulses the reflections from ob- 'y jects under scrutiny are received in the same an tenna. By the observation of detected pulses, for example, on the screen of a cathoderay oscilloscope (not shown) the distance to the Areflecting Y object may be determined and when coordinatedy with the orientation of the antenna the direction thereof can also be ascertained as is stood in the art. y

well undere.

The ultra-high frequency radio waves are gen.- 1

erated by a transmitter II which is coupled to the antenna I by a coaxial transmission line I2. (Whenoperating at higher frequencies a wave guide may be used in place ofthe coaxial line I2.)

The operation of the transmitter I I is controlledby a modulator I3 that produces positive direct current pulses of the required lengthand recurrence rate, The general nature of these pulses is indicated by the curve that appears'in the-drawing immediately above the modulator I3 and in which Voltage is plotted against time. These direct currentpulses are impressed on the transmitter I I and cause the production of ultra-high frequency radio waves during each pulse. l

A radio receiver is coupled to the coaxial line I2 through the duplexing unit. The duplexing unit comprises a resonant cavity I4 tuned'to the frequency of the radio oscillations produced by the transmitter-II with a gas discharge device I5 mounted therein. The gas discharge tube I5 comprises two electrodes- I6- connected to the walls of the cavityv I4 vand enclosed in a partially evacuated vessel I1. The cavity 'I4' isrooupled to the coaxial line I2 by means of an aperture I8 in the common wall of the outer conductor of the line I2V and the cavity. A second aperture I9 in the opposite wall of Vthe cavity provides 'coupling to the radio receiver.

During the transmission of pulses vof radio waves by the transmitter I I a voltage Vsufficiently high to break down the gap betweenthe electrodes I6 is developed in the resonant cavity I4. As a result the path from the coaxial lin'e I2 to the radio receiver has a very low'impedance' 474,164,V filed January 30, 1943, and A. L. Samuel Serial N0. 474,122, filed January 30, 1943.

The radio receiver comprises a converter 20 the intermediate frequency output of which is coupled rendered operative during the period of trans? through a coaxialline 29 to a preliminary intermediate frequency amplifier 34.A The output of the intermediate frequency amplifier 30 is branched Vbetween the final intermediate frequency amplifier, detector andindicator circuit 38 andan automatic frequencycontrol circuit 4|).

The converter 20 employs a. tuned coaxial inq put line 2| which is coupled at one end to the resonant cavity I4 vby means of the aperture I9.

Y. capacitor coupling plate 25.

At the other end of the line 2I there is provided a crystal detector or modulator 22 connected to outer conductor of the line 2| through a radio frequency by-pass capacitor 23 and to the inner conductor` by means of theY usual contact point. Heterodyning'oscillations generated in the beating oscillator 24 are introduced by means of the The intermediate frequency output of the converter is taken off across the capacitor 23 by means of the coaxial line 29.

The preliminary intermediate frequency amplifier 3'0 comprises one or more initial stages 3| and a subsequent stage employing a vacuum tube amplifler 32. This stage is conventional except that provisions are, made for supplying a blocking bias 'between the cathode and control grid through the low-pass filter 33. A voltage for blocking the amplifier tube 32 during the initial portion of transmitted pulses is supplied through thel connection 34 vas will be later described.

The output of the amplifier 32 is divided between two branches. In the branch38 the signal is further amplified at the intermediate frequency level, detected, amplified at the signal or video level and impressed on the indicator. The automatic frequency control circuit 40 is connected in the other branch.

The automatic frequency control circuit comprises a gating amplifier tube 4I and a frequency discriminatorl circuit 42. Obviously additional intermediate frequency amplifier stages may be used ahead of -the discriminator or direct current i amplifier stages may be employed in the discriminator output orrhothcasrequired by .crcuit'confv/ ditions. VAs is well understood in the art, the frequency discriminator 42 with its associated integrating circuit responds to the intermediate frequency input to produce a direct current output depending in amplitude and polarity upon the frequency of the input. When this frequency is of the correct value there is no direct current out-Y put and as the frequency varies from the desired value the output' voltage also varies being of opposite signs for frequenciesV above 'and below the required value. This direct current output is supplied through the connection 43 to the beating oscillator 24 to regulate Vthe frequency of the oscillations produced thereby, for example, by the control of the voltage of the repeller electrode of a velocity-modulation oscillator of the reflection type.`

The amplifier 4I is normally blocked by reason of the fact that no bias voltage is supplied to its screen grid. However, during the transmission of pulses amplifier 4I is rendered operative by l virtue of a positive direct current voltage applied to its screen grid through the connection 44 and the resistance-capacity filter 45. This voltage isk obtained from the Vvoltage divider 46 connected across the output of the modulator I3. In this way the automatic frequency control circuit 4I)u is mission of pulses and tunes the beating oscillator 24 to make the receiver selective to signals of the frequency of the transmitted pulses. The time constant of the output circuit of the discriminator 42 is such that this regulation of the beating oscillator 24 is maintained throughout the period between pulses.

A second voltage divider 35 is connected across the output ofthe modulator I3. The low voltageY tap on the voltage divider 35 is connected through the capacitor 36 to the lead 34 to the cathode of they tube 32. As a result-of the differentiating action of the capacitor 36 a short pulse ispro'- duced during the initial portion of each transmittedpulse` This short pulse is applied through the connection34 and the filter 33 upon the cathodeof the tube 32 making the cathode suiciently ositive with respect to the grid to block the tube 2. stages of the receiver and particularly the automatic frequency control circuit til are rendered unresponsive to receiver inputs duringthisinitial period of eachA transmitted pulse. The gap of the discharge tube i will have suflicient time to brealr down` during this period and during the remainder oi the transmitted :pulse the broken dovvny gap will produce a high attenuation of theV input to the receiver. As a result the oscillatory energy to the receivenvvill be at a suinciently low level to: cause normal operation ofthe automatic volume control circuit is without such: spurious effects: and false responses as might be produced by the high energy input that otherwise occur during the initial period ofthe pulse beforeV the discharge between the electrodes It has taken place.

The `embodiment of the drawing is shown as illustrative of the applicationof the invention and is designed to meet certain operating and circuit conditions and obviously is susceptible of various modifications without departing frornthe invention. I wish to be. understood that I do notcdesire to be limited to the exact details of construction shown and described for obvious modications vvill occur to a personskilled in the art. For example: othe conditions might nl alre it desirable to apply the-blocking bias to the rst intermediate frequencyampliner stage instead of a later stage such as tube 32. Similarly, both the blocking bias for the tube 32 and the gating bias for the tube 4 IlA might under certain conditions be applied to electrodes other than those used in this circuit with similar ormore favorable results.

f What is` claimed is:

1. In a pulse transmitting and receiving system employing a duplexing unit operated by the transmitted pulses to reduce the input to the receiver during transmission, means for producing a control pulse at the beginning of the transmitted pulse and of shorter duration, means responsive to said control pulses for blocking the receiver, a normally inoperative automatic frequency control responsive to the receiver output, and means for rendering said automatic frequency control op erative during each transmitted pulse.

2. In a pulse transmitting and receiving system employing a duplexing unit including a dischargegap broken down by the high voltage of the transmitted pulses to reduce the input to the receiver during transmission, means for producing a control pulse at the beginning of each transmitted pulse and of shorter duration, means responsive to said control pulses for blocking the receiver, a normally inoperative automatic frequency control responsive to the receiver output, and means for rendering said automatic frequency control operative' during each transmitted pulse.

3. In a pulse reflection system, a transmitter for producing recurrentpulses of radio frequency oscillations, a radio receiver comprising a beating oscillator, a rst detector for combining the radio input to said receiver with the output of said oscillator and an intermediate frequency amplifier for amplifying the output of said first detector, an antenna, connections from said antenna to the output of said transmitter and the input to fAs a result of this action the'subsequentV 6,. said receiver, duplexingmeans included inV said connections and operated by the output of said transmitter for producing a high attenuation in the input to said receiver, an automatic frequency control for said beating oscillator responsive to the output of said intermediate frequency ampliner and normally inoperative, means for rendering said automatic frequency control operative during the transmission of pulses by said trans mitter, means for generating a control pulse beginning at substantially the same time as each of said recurrent pulses and of shorter duration, and means for applying said control pulse to said intermediate frequency amplifier to block itstransmission.

4. In a pulse reflection system, a transmitter for producing recurrent pulses of radio frequencyV oscillations.. a radio receiver comprising a beating oscillator, a first detector foi` combining the radio input to said receiver with the output of said oscillator and an `intermediate frequency amplifier for amplifying the output of said detector, an antenna, connections from said antenna to the output of said transmitter and the input to said receiver, duplexing means included in said connections and comprising a discharge gap broken down by the 1nigh voltage developed thereacross during the transmission of pulses by said transmitter to introduce a high attenuation in the input connections to said receiver, an automatic frequency control for said beating oscillator responsive to the output of said intermediate frequency ampliiier andnormally inoperative, means for rendering said automatic frequency control operative during the transmission of said recurrent pulses, means for generating a control pulse beginning at substantially the same time as each of said recurrent pulses but of shorter duration, and means responsive to said control pulse for blocking said intermediate frequency amplifier.

5. A system according to claim 3 in` WhichA said intermediate frequency amplifier comprising a vacuum tube having a cathode and a VcontrolV 6. In .a pulse reiiection system, an ultra-highA frequency radio transmitting oscillator, a generator of recurrent signal pulses, connections for supplying said pulses to said transmitting oscil lator to cause the production of oscillations during each of said signal pulses, a radio receiver comprising a beating oscillator, a first detector for combining the radio input to said receiver with the output of said beating oscillator, an intermediate frequency amplifier for amplifying the output of said detector, an automatic frequency con trol system responsive to the output of said intermediate frequency ampliiier for regulating said beating oscillator and normally inoperative, means responsive to said signal pulses for rendering said automatic frequency control operative during each said signal pulse, an antenna, connections from said antenna to the output of said transmitting oscillator and to the input to said radio receiver, a duplexing unit responsive to outputs from said transmitting oscillator for producing a high attenuation in the input to said radio receiver, a differentiating circuit having an input connected to said generator of recurrent signal pulses to produce control pulses of a shorter length thansaid signal pulses beginning at the same time as said signal pulses, and connections for supplying said control pulses to said intermediate frequency amplifier to block its transmission.

7. In a pulse reflection system a radio transmitting oscillator, a generator of recurrent signal pulses, means for modulating said oscillator by said signal pulses to cause the transmission of radio waves during each signal pulse, a radio receiver comprising a-beating oscillator, a rst detector for combining the radio input to said receiver with the output of said beating oscillator and an'intermediate frequency amplifier for amplifying the output of said rst detector, an antenna, connections from said antenna to the output of said transmitting oscillator and to the input to said radio receiver, a duplexing unit included in said connections and comprising a discharge gap responsive to the high voltage produced by the output of said transmitting oscillator to produce a high attenuation in the input to said receiver, a normally inoperative automatic frequency control system having an input connected to the output of said intermediate frequency amplier and an output connected to said beating oscillator, means responsive to said signal pulses for rendering said automatic frequency control system operative to regulate said beating oscillator in response to the output of said intermediate frequency amplifier, a differentiating circuit having an input connected to said signal generator to produce control pulses of shorter duration than said signal pulses beginning at the same time as said signal pulses, and connections for supplying said control pulses to said intermediate frequency ampliiier to block its transmission.

8. In a pulse reilection system, a transmitter for vproducing recurrent pulses of radio frequency oscillations, a radiov receiver comprising a beating oscillator, a first detector for combining the output of said beating oscillator with the radio input to said receiver and an intermediate frequency amplifier for amplifying the output of said first detector, an antenna, connections from said antenna to the output of said transmitter and to the input to said radio receiver, a duplexing unit included in said connections and com- 8, prising a discharge gap'responsive to the output of said transmitter for producing a high attenuation in the input toY said radio receiver, means for producing control pulses' of shorter length I than said recurrent pulses beginning at the same time as each of said recurrent pulses, and connections for supplying said control pulses to said intermediate frequency amplier to block its transmission.

9. In a pulse reection system, a radio transmitting oscillator, a generator of recurrent sig-l nal pulses, means for modulating said transmitting oscillator by said signal pulses' to cause the transmission of radio waves during each signal pulse, a radio receiver comprising a beating oscillator, a rst detector for combining the radio input to said receiver with the output of said beating oscillator and an intermediate frequency amplifier for amplifying the output of said first detector, an antenna, connections for said antenna to the output of said transmitting oscillator and to the input to said radio receiver, a duplexing unit included in said connections and comprising a discharge gap responsive to the output of said transmitting oscillator to produce a high attenuation in the input to said radioA receiver, a differentiating circuit having an input connected to said generator of recurrent signal pulses to produce control pulses of shorter duration than said 'z signal pulses and beginning at thesame instant,

and means for supplying said control pulses to said intermediate frequency amplier to block its transmission.

STEWART E. MILLER.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS A 

